Fix phpstan/phpstan#14411: Incorrect type narrowing with dependent types

src/Analyser/MutatingScope.php

@@ -3603,6 +3603,15 @@
 			}
 
 			foreach ($variableTypeGuards as $guardExprString => $guardHolder) {
+				if (
+					array_key_exists($exprString, $theirExpressionTypes)
+					&& $theirExpressionTypes[$exprString]->getCertainty()->yes()
+					&& array_key_exists($guardExprString, $theirExpressionTypes)
+					&& $theirExpressionTypes[$guardExprString]->getCertainty()->yes()
+					&& !$guardHolder->getType()->isSuperTypeOf($theirExpressionTypes[$guardExprString]->getType())->no()
+				) {
+					continue;
+				}
 				$conditionalExpression = new ConditionalExpressionHolder([$guardExprString => $guardHolder], $holder);
 				$conditionalExpressions[$exprString][$conditionalExpression->getKey()] = $conditionalExpression;
 			}

tests/PHPStan/Analyser/nsrt/bug-14411-regression.php

@@ -0,0 +1,36 @@
+<?php // lint >= 8.0
+
+declare(strict_types = 1);
+
+namespace Bug14411Regression;
+
+use function PHPStan\Testing\assertType;
+use function PHPStan\Testing\assertVariableCertainty;
+use PHPStan\TrinaryLogic;
+
+interface OrderInterface {}
+
+class Event
+{
+	/** @return mixed */
+	public function getSubject()
+	{
+		return new \stdClass();
+	}
+}
+
+function getOrder(Event|OrderInterface $event): OrderInterface
+{
+	if ($event instanceof Event) {
+		$order = $event->getSubject();
+		assert($order instanceof OrderInterface);
+	}
+
+	if ($event instanceof OrderInterface) {
+		$order = $event;
+	}
+
+	assertVariableCertainty(TrinaryLogic::createYes(), $order);
+
+	return $order;
+}

tests/PHPStan/Analyser/nsrt/bug-14411.php

@@ -0,0 +1,34 @@
+<?php // lint >= 8.0
+
+declare(strict_types = 1);
+
+namespace Bug14411;
+
+use function PHPStan\Testing\assertType;
+
+/** @phpstan-impure */
+function get_mixed(): mixed {
+	return random_int(0, 1) ? 'foo' : null;
+}
+
+/** @phpstan-impure */
+function get_optional_int(): ?int {
+	return random_int(0, 1) ? 42 : null;
+}
+
+function (): void {
+	$a = get_mixed();
+
+	if ($a !== null) {
+		$b = $a;
+	}
+	else {
+		$b = get_optional_int();
+	}
+	if ($b !== null) {
+		assertType('mixed', $a);
+		if ($a === null) {
+			echo 'this is absolutely possible';
+		}
+	}
+};